Nucleic Acids

Give 2 examples of nucleic acids

- DNA

- RNA

What monomer makes up nucleic acids?

- Nucleotides

What are nucleotides made from?

- Phosphate

- Pentose sugar

- Organic/ nitrogenous base

What do lots of nucleotides form?

Polynucleotide chain/ strand

What bond holds together nucleotides?

- Phosphodiester

What is a common name for the polynucleotide chain that doesn't include the bases?

Sugar-phosphate backbone

What reaction joins nucleotides?

Condensation reaction

How do polynucleotide chains differ?

Sequence of bases in the polynucleotide chain

What pentose sugar does DNA have?

- Deoxyribose

What pentose sugar does RNA have?

- Ribose

What are the 4 bases found in DNA? Group them

Purines - Adenine, Guanine

Pyrimidines - Cytosine, Thymine

How are polynucleotide chains arranged in DNA?

- Antiparallel

- Twist to form a double helix

- Held by hydrogen bonding (between bare pairs)

How many hydrogen bonds do each base pair form?

Adenine forms 2 with Thymine

Guanine forms 3 with Cytosine

Explain DNA stability

- double helix with many hydrogen bonds

- strong due to phosphodiester bonds in sugar-phosphate backbone

Explain complementary base pairing for DNA

allows DNA to be replicated exactly and the weak hydrogen bonds allow the 2 strands to seperate

Explain DNA being compact

- DNA is long so the double helix shape allows DNA to fit inside the nucleus

Explain genetic code

determined by base sequence of DNA which controls proteins synthesis

Name the bases in RNA

- Adenine, Guanine, Cytosine, Uracil (instead of thymine)

How can analysis of a piece of DNA tell you it is single stranded?

- Ratio of base pairs wouldn't be 1:1

- Different number of base pairs of A:G and C:T

- Different number of purines and pyrimidines

How does DNA replicate? Overview

- Use semi conservative replication

- The new DNA molecule contains 1 parent strand and 1 new strand

Explain in full detail how DNA is replicated

- By semi-conservative replication

- The enzyme DNA helicase unwinds the double helix DNA molecule into 2 polynucleotide strands by breaking the weak hydrogen bonds between complementary base pairs

- The exposed strands act as a template for the formation of a new DNA strand

- New DNA nucleotides (present in nucleus) are attracted to the exposed template strand by complementary base pairing to form hydrogen bonds between the bases

- The enzyme DNA polymerase joins the new nucleotides together to form a new polynucleotide strand. Phosphodiester bonds are formed between adjacent nucleotides via condensation reactions.

Describe the experiment for finding evidence of semi-conservative replication

- Generation 0 - DNA grown in heavy isotope 15N then transferred to 14N medium - both heavy strands

- Generation 1 - One division taken place - 2 hybrid molecules (1 strand 14N 1 strand 15N)

- Generation 2 - Second division taken place - 2 hybrid molecules and 2 light molecules

- Generation 3 - third division takes place - 2 hybrid molecules and 6 light molecules

Lighter float higher up

- Different bands show 14N acts as a template strand

Describe the structure of ATP

- Ribose pentose sugar

- Adenine organic base

- 3 Phosphate groups

- They are derived from nucleotides

Describe the synthesis of ATP

- Formed from ADP and Pi in a condensation reaction catalysed by the enzyme ATP synthase

- The addition of the phosphate is called phosphorylation (in a condensation reaction?)

- Energy input is required for the reaction to take place

Describe the breakdown of ATP

- Energy mainly stored in the bond between the 2nd and 3rd phosphate groups

- This is because it is unstable and easily broken by the enzyme ATP hydrolase in a hydrolysis reaction

- This removes the phosphate group and releases energy when ATP becomes ADP and Pi

How is ATP used in active transport?

- Energy is required to move substances against their conc gradients using carrier proteins in plasma membranes

- These are also ATP hydrolase enzymes as they catalyse the splitting of ATP into ADP and Pi

- The energy released is used to change the shape and pump the molecule

How is ATP used in exocytosis?

- Energy is requires to make vesicles which is membrane bound spheres synthesised by the golgi body and back large molecules before they are secreted

- Energy is also required for the opposing process, endocytosis which brings large molecules into cells

How is ATP used in synthesis of substances? (Biosynthesis)

- Energy is required to make large molecules (polymers) from smaller ones (monomers)

- eg synthesis of proteins by condensation reactions of amino acids or DNA molecules by condensation of nucleotides before cell division

How is ATP used in movement?

- Energy required for contraction of muscles

How is ATP used in activation of other molecules?

- ATP transfers its phosphate group to other molecules by phosphorylating them to make them more reactive

- required hydrolysis of ATP into ADP + Pi

- eg glucose requires phosphorylating at the start of respiration to reduce the activation energy of the reaction

Describe ATP as an energy source?

- Good as an immediate energy source used by cells for metabolic reactions

- Hydrolysed for energy release

- Not good as a long term energy source due to its instability of its phosphate bonds

- Cells only maintain a few seconds supply so is good as the immediate energy source

- ATP can be reformed rapidly from ADP and a phosphate/Pi

Why is ATP more useful than glucose?

- ATP breakdown is a single reaction so energy is immediately available but glucose is a complex breakdown which requires several stages

- ATP is soluble so moves around inside cells easily but not through membranes

- ATP breakdown releases a small amount of energy which is ideal for the reactions that require energy compared to glucose which releases lots of energy at one time (energy is wasted as heat)

Describe DNA in nuclei of humans

- 23 homologous pairs

- Each pair consists of one maternal and one paternal chromosome

- 46 chromosomes total

When do chromosomes become visible?

- When chromatin starts to form chromosomes and they shorten and thicken as the DNA coils up around histone proteins

Define chromosome

- long, linear structure consisting of DNA and histone proteins (form chromatin)

Define homologous chromosomes

pair of chromosomes containing the same genes in the same positions (loci) each derived originally from a parent's gamete at fertilisation

Define chromatid

one of the 2 threads of the chromosome formed after DNA replication

Each chromatid is an exact replica of the other (sister chromatids)

What holds 2 chromatids together?

centromeres

Describe mitosis

- parent cell divides to form 2 daughter cells which contain the exact copy of the DNA of the parent cell

- they are genetically identical

- replication of DNA takes place prior to mitosis

What does mitosis do?

- Increases the number of cells during growth

- Replaces tissue during repair

- Allows asexual reproduction so have genetically identical offspring

What is the only way in which variation can arise when cells divide by mitosis

- Mutation

Name the phases of mitosis and what happens before it

- Prophase

- Metaphase

- Anaphase

- Telophase

Interphase occurs before it to make preparations for the division

Describe interphase

- Cell is actively synthesising proteins

- Chromosomes are not visible before mitosis

- DNA has been replicated

- Increase in number of organelles

Describe prophase

- Nuclear membrane breaks down

- Nucleolus disappears

- DNA free in cytoplasm

- Chromosomes condense and become shorter so visible

- Centrioles divide and move to the poles of the cell

- Spindle fibres form across the cell

Describe metaphase

- Chromosomes move to the equator of the cell

- Chromosomes attach to the spindle fibres at their centromere

Describe anaphase

- Spindle fibres contract

- The centromeres that held the chromatids together divide

- Sister chromatids are pulled apart

- Sister chromatid from each chromosome moves to opposite poles of the cell

Describe telophase

- Nuclear membrane forms around each group of chromosomes

- Cell has 2 nuclei briefly

- Nucleolus reforms

- Chromosomes unwind and diffuse so no longer visible

What marks the end of mitosis?

- Division of cytoplasm - cytokinesis begins

Describe cytokinesis

- Division of cytoplasm follows mitosis very quickly

- Cells without cell walls pinch themselves in 2 and a membrane forms around the middle of the cell

- The cell membrane from one side of the cell joins the other and 2 new daughter cells separate

- Cells with a cell wall cannot pinch themselves in 2, instead a new cell wall forms around the middle of the cell

Name and describe the parts of the cell cycle

- G1 - cells prepare for DNA replication

- S - DNA replication takes place

- G2 - short gap before mitosis

- M - mitosis

What parts of the cell cycle make interphase?

- G1, S and G2

What does it mean when cells have shorter interphase?

- divide more frequently

What does it mean when cells have longer interphase?

- divide less frequently

Describe neurons interphase?

- G0 phase

- don't have the ability to divide

How do you find mitotic index?

number of cells undergoing mitosis/ total number of cells

- expressed as % or decimal depending on what the question asks for

How do prokaryotic cells divide? Why not mitosis?

- Binary fission

- No nucleus

Describe the process of how prokaryotes divide

- Replication of circular DNA including plasmids

- Cell membrane begins to grow between the two main DNA loops and begins to pinch inwards dividing the cytoplasm in 2

- New cell wall forms between 2 DNA loops, forming 2 daughter cells, each with an exact copy of the main DNA loop and a variable number of copies of plasmids

How do viruses replicate?

- They are non living so can't undergo cell division

- Instead they replicate by attaching to a host cell using their attachment proteins on their surface. They then inject their nucleic acids into the host cell which then produces viral components (proteins) that assemble into new viruses

Give an overview to cancer

- Mitosis is controlled process

- 'Mass of abnormal cells'

- Cancer is from uncontrolled, rapid cell division caused by mutations in the genes that control cell division

- This forms a mass called a tumour

- Cancerous tumours don't respond to signals from nerves and hormones as normal healthy cells would

- So don't undergo programmed cell death (apoptosis) which normally takes place when damage is detected

Give an example of how cancer may develop

- A protein, p53 normally prevents damaged or faulty cells from dividing and forming faulty daughter cells

- The p53 gene codes for the p53 proteins

- Mutation of the p53 gene means no protein is made/ non functional

- The faulty cell can continue to divide when completing the cell cycle

- Uncontrolled cell division produces cancer

We can say if there is a mutation in one gene that codes for a protein, it may not have an effect if the organism is diploid as the same gene would be in the other homologous chromosome

How do cancer treatments work?

- Blocking parts of the cell cycle to control rate of cell division. They disrupt this by:

- Prevent DNA from replicating

- Prevent spindle fibre formation or another stage of mitosis

What problems do cancer treatments have for other cells?

Disrupt cell cycle of normal cells

Why are cancer cells more prone to damage by cancer drugs than ordinary cells?

- Faster rate of division compared to ordinary cells

- Cancer drugs more effective against rapidly dividing cells

Why do people undergoing cancer drug treatment frequently lose their hair? chemotherapy

- Hair producing cells divide rapidly so are vulnerable to damage

Genetic code is degenerate and non overlapping - give definitions of the two

degenerate - more than one base triplet codes for each amino acid

non-overlapping - each base is part of only one triplet